1. Field of the Invention
The disclosed technology relates to non-volatile resistive memory devices, such as for example RRAM, PCM, or other, and methods for biasing the resistive memory structures of such non-volatile resistive memory devices for electroforming, setting, writing/write-assisting and the like.
2. Description of the Related Technology
Non-volatile resistive memory, e.g. resistive random access memory (RRAM), interfacial switching memory, phase-change memory (PCM), or other, is emerging as a disruptive memory technology, implementing memory function in a resistance (rather than stored charge), the value of which can be changed by switching between a low and a high level. Although the phenomenon of reversible resistance switching has been studied since the 1960s, recent extensive research in the field has led to the proposition of several concepts and mechanisms through which this reversible change of the resistance state is possible. Many non-volatile resistive memory concepts, e.g. most RRAM, are based on a metal-insulator-metal (MIM) structure in which a conductive path is created or dissolved, corresponding to low-resistive and high-resistive states. This attribute is associated with a high scalability potential, beyond the limits currently predicted for flash memory. The MIM resistive memory structure is connected serially with e.g. an nMOS transistor, which acts as a cell selector.
A suitable insulating material for RRAM is for example a thin HfO2 dielectric film. Sandwiched between two conducting electrodes, such a film exhibits resistive switching properties, which can be either “unipolar” switching or “bipolar” switching, depending on, for example, the materials used as electrodes and on the method used to deposit the active (oxide) film. As used herein, the term “bipolar” switching is used to describe a memory cell which switches from a high resistance state to a low resistance state under a first electrode polarity, while switching from a low resistance state to a high resistance state under a second electrode polarity that is opposite to the first polarity. In contrast, as used herein, the term “unipolar” switching is used to describe a memory cell which switches from a high resistance state to a low resistance state under one electrode polarity, while also switching from a low resistance state to a high resistance state under the same polarity. Without being bound to any particular theory of operation, the bipolar operation of HfO2, requiring voltages of opposite polarity to switch on/off the cell, is believed to be due to the formation of conductive paths (filaments) associated with presence of oxygen vacancies (VO), which can be ruptured/restored through oxygen/VO migration under electric field and/or locally enhanced diffusion. The formation of the filament (forming, or electroforming) is believed to take place along pre-existing weak spots in the oxide, for instance along the grain boundaries in case of a polycrystalline HfO2, which presumably have larger amount of defects and also a higher oxygen diffusivity compared to the bulk of the material. Other suitable insulating materials for this type of memory include HfO2, zirconium dioxide, titanium dioxide, tantalum dioxide/ditantalum pentoxide.
For some biasing schemes applied to such non-volatile resistive memory devices, such as for example electroforming for a first formation of the resistive memory element, for example for writing/setting and erasing/resetting, a higher voltage may be m over the resistive memory element, above the supply voltage and/or above the transistor gate oxide breakdown voltage. The electroforming voltage, for example, is typically the highest voltage which is applied to the cell and is needed only once, to get the cells ready for operation. In contrast, the Set (on-switching) and Reset (off-switching) voltages are lower, but still a voltage above the supply voltage may be used.